Traditional net weight scales produce a dispersion of weights about a target weight. This dispersion is typically a bell shaped curve, approximating a binomial distribution. Because of the random nature of these scales, government and corporate standards for filling by weight have typically allowed for a nominal number of below-label-weight packages or containers.
In order to comply with package weight standards in the United States, and other countries, the traditional net weight scale is set so that the target weight is the label weight of the package, plus an amount to insure that most, typically 95% or 2-sigma, packages have at least the weight printed on the container. This concept is generally illustrated in the histogram in FIG. 1. Thus, the regulations for package weight accuracy take into account the fact that traditional net weight scales are random in nature and that occasional packages below-label-weight will be produced.
In order to accommodate the packaging of items having varying weights, such as frozen vegetables, combination weighers have been heretofore developed and are being routinely used. These combination weighers operate on a principle that does not produce random weights dispersed like those shown in FIG. 1. Rather, combination weighers employ a number of hoppers associated with check weighers (typically 10 to 20) which are scanned to determine which hoppers, when discharged together, will deliver a total weight equal or closest above a preset target weight. One method of achieving the combination process is described in U.S. Pat. No. 4,267,894. Typically, the target weight of the machine is set to the label weight of the final package to be filled. The weight distribution for a typical combination weigher is depicted generally in the diagram identified as FIG. 2.
Combination weighers comprise, in typical operations, a multiplicity of hoppers, each of which is connected with a check weigher. Each hopper is equipped with an unloading gate and associated with a refilling mechanism. Assuming that all of the hoppers have items in them, the respective check weigher signals are summed according to an algorithm that provides various combinations of signals. The combination or sum of check weigher signals that represents the weight equal to or greater than a minimum weight, but less than or equal to a maximum weight is determined. The hoppers corresponding to the selected combination of check weigher signals are discharged by operating their unloading gates, thereby dropping the items from the selected hoppers into a discharge chute, which typically is interfaced to a packaging machine. When the unloading gates close, the hoppers that were emptied are refilled with items.
A simplified diagram of the signals generally used in a typical combination weigher is shown in FIG. 3, with respect to a typical combination selection and machine control logic. The input signals are from check weighers, designated S1 . . . Sn, the minimum weight set-point, the maximum weight set point, and the cycle start. The output signals are the unloading gate signals, designated U1 . . . Un, the selected weight signals, and the cycle complete signal. A simplified timing diagram, as shown in FIG. 4, indicates the sequence of operations of a combination weigher. It is assumed that the hoppers have items in them ready for weight measurement by the check weighers. The cycle start signal is received at T.sub.0. The combination selection and machine control logic then proceeds to find the best combination of check weigher signals within the group S.sub.1 . . . S.sub.n, that is within the minimum weight set-point while not exceeding the maximum weight set-point. When the best combination is found, at time T.sub.1, the selected weight output (the combined weight of the selected check weighers) is considered to be valid, whereupon the hoppers corresponding to the selected check weighers are unloaded by giving output signals U1 . . . Un, and the cycle complete signal goes to a true setting. During the time from T.sub.1 to T.sub.2, the empty hoppers are refilled with items. Then, the entire cycle is repeated at time T.sub.2 with the next cycle start signal. The timing is typically more complex than described herein in conventional combination weighing operations. Many applications require two or more cycles to be interleaved to obtain more speed. For example, while one set of hoppers is being refilled, the remaining check weighers whose hoppers have items are scanned and selected by the combination selection and machine control logic. U.S. Pat. Nos. 4,385,671, 4,441,567, and 4,470,166 describe one approach to obtain speed in this manner. Regardless of the complexity of the timing to obtain speed, the methods used herein apply and the combination selection and machine control logic is in the prior art.
The method of obtaining the combination of signals from the check weighers for minimum and maximum weight set points is irrelevant. The first implementations of combination weigher systems used a selection process that stopped searching at the first combination of weights within the minimum and maximum set-points, as for example, see U.S. Pat. Nos. 3,939,928 and 4,336,852. This method, while easy to implement, did not provide the best performance for the end user who is striving for the weight closest to the lower weight set-point. U.S. Pat. Nos. 4,267,894, 4,454,924 and 4,466,500 describe methods to find the best weight which is the recommended approach.
Thus, the prior art combination weigher systems in general, have, at most, two user settable weight parameters, the minimum weight set-point and the maximum weight set-point. However, many applications of combination weighers require several user settable weight zones with a means to preset the frequency of weighments within the zone. Additionally, many users desire a means whereby the combination selection and machine control logic determines the optimum weight set-point automatically so that the average weight of the discharged items does not go below a predetermined weight. Finally, many users want a minimum number of discharges to occur between the selection of various weight zones. Thus, the prior art combination weigher systems do not enable a control of dispersion of package weight. For example, if the target weight of a combination weigher is set below the label weight of the package or container, a disproportionate and uncontrolled number of packages will be below the label weight. When the individual weight of the items of product is high relative to the total weight of the package, the number of below weights becomes unacceptable. The effect of setting target weights below the label weight is, therefore, not successfully practiced with present combination weigher systems.
In addition, the prior art combination weigher systems have not provided the opportunity to specifically control discharge weights; nor have they permitted maintaining average weight from the systems; nor have they provided a control of the percentage of discharges at weights below the target weight; nor provided for the computation of set-points.
In summary, the prior art combination weighers have provided a combination selection and machine control logic which accepted signals from check weighers and permitted the setting of maximum weight set-points and minimum weight set-points, along with output signals for discharging the items in hoppers into a chute for feeding into an interfaced packaging machine. The prior art, of course, has provided for cycle start signals and cycle complete signals. However, these limited combination weighers have left much to be desired for current users.
A principal object of the present invention is the provision of an improved combination weighing system.
Another object of the invention is the provision of improved combination weighing methods and apparatus which can be made to conform to desired criteria set by the user.
A further object of the invention is the provision of an improved combination weighing system which sets inputs to a combination selection and machine control logic so as to compare the performance of the combination weighing system with a desired set of parameters determined by the user.
An additional object of the invention is the provision of an improved combination weighing system that discharges weighments below a set point but, at the same time, controls the number of weighments therebelow.
A still further object of the invention is the provision of an improved combination weighing system whereby the average weight of the discharges into the chute is not less than a set weight.
Still another object of the invention is the provision of an improved combination weighing system which permits the control of discharge weights relative to a set point so as to occur at specified intervals in the operation of the system.
A still further object of the invention is the provision of an improved combination weighing system which permits the automatic determination of a set point which is greater than the minimum acceptance weight set for the system.
Various additional objects and advantages of the invention will become apparent by reference to the following drawings and description.